In general, phase change inks are solid at ambient temperatures and liquid at the elevated operating temperatures of an ink jet printing device. Liquid phase ink jet droplets are ejected from the printing device at an elevated operating temperature and, when the ink droplets contact the surface of a substrate, they quickly solidify to form a predetermined pattern.
Phase change ink is advantageous for printing purposes since it remains in a solid phase at room temperature during shipping, long-term storage, etc. Also, problems associated with nozzle clogging due to ink evaporation are largely eliminated, thereby improving the reliability of ink jet printing. Furthermore, since the ink droplets solidify rapidly upon contact with the substrate, migration of ink along the printing medium is greatly reduced and image quality is improved. Rapid solidification allows high quality images to be printed on a wide variety of printing media.
Early references to phase change inks for ink jet printing involved monochrome inks jetted by electrostatic printing devices. Thus, for example, U.S. Pat. No. 3,653,932 discloses a low melting point (30.degree. C. to 50.degree. C.) ink having a base comprising di-esters of sebacic acid. In a similar process, U.S. Pat. No. 3,715,219 describes low melting point (30.degree. C. to 60.degree. C.) inks including a paraffin alcohol-based ink. One disadvantage of printing with low melting point phase change inks is that they frequently exhibit offset problems. Specifically, when substrates printed with these inks are stacked and stored for subsequent use, the ink adheres to adjacent surfaces, particularly if the printed substrates are exposed to high ambient temperatures.
Phase change inks are well known in the art. U.S. Pat. Nos. 4,390,369 and 4,484,948 describe methods for producing monochrome phase change inks that employ a natural wax ink base, such as Japan wax, candelilla wax, and carnauba wax, which are subsequently printed from a drop-on-demand ink jet device at a temperature ranging between 65.degree. C. and 75.degree. C. U.S. Pat. No. 4,659,383 discloses a monochrome ink composition having an ink base including a C20-24 acid or alcohol, a ketone, and an acrylic resin plasticizer. These monochrome ink compositions are not durable and, when printed, may become smudged upon routine handling and folding.
Japanese Patent Application No. 1,280,578 discloses the use of aliphatic and aromatic amides that are solid at room temperature, such as acetamide, as printing inks. U.S. Pat. No. 4,684,956 is directed to monochrome phase change inks utilizing synthetic microcrystalline wax (hydrocarbon wax) and microcrystalline polyethylene wax. This molten composition can be applied to a variety of porous and non-porous substrates using drop-on-demand ink jet application techniques.
European Patent Application Nos. 0287352 and 0206286 disclose phase change ink jet printing in color. The ink bases for these systems include fatty acids, a thermoplastic polyethylene and a phase change material in the first application; and the alcohol portion of a thermosetting resin pair, a mixture of organic solvents (o- and p-toluene sulfonamide) and a dye in the second application.
Several prior art references disclose manipulation of printed images formed from phase change inks, either during or following the printing process. In U.S. Pat. No. 4,745,420, droplets of a phase change ink are ejected onto a target and subsequently spread by the application of pressure to increase the coverage and minimize the volume of ink required. In other words, droplets of phase change ink that do not initially cover the entire target are spread over the entire target surface by application of pressure.
Similarly, in xerographic image fusing, the area of contact between the toner and the substrate is substantially increased by causing the toner to spread and penetrate somewhat into the underlying substrate. See Williams, "The Physics and Technology of Xerographic Processes," J. Wiley & Sons (1984). The mechanical properties of the toner are such that plastic deformation and flow occur rapidly. In both of the aforementioned references, the ink or toner spreads across the paper, forming opaque characters or patterns thereon.
Although the previous references describe fusing of images between a pair of mechanically loaded rollers at ambient temperatures, hot roll fusing has also been used in toner applications. In hot roll fusing, two rolls (one heated) are mechanically loaded together and rotated to provide transient application of heat and pressure to the substrate. The toner is typically heated to above its glass transition temperature (T.sub.g), which enables it to coalesce, flow, and penetrate the substrate. Rolling pressure and capillary action facilitate coverage. See Dr. John W. Trainer, "Trends and Advances in Dry Toner Fusing," Institute for Graphic Communication (June 1985).
Another system for applying phase change inks, described in U.S. Pat. No. 4,751,528, relates to an ink jet apparatus for the controlled solidification of phase change inks to assist in controlled penetration of the substrate. This apparatus includes a substrate-supporting, thermally conductive platen as well as a heater and a thermoelectric cooling arrangement, both disposed in heat communication with the platen.
Ink jet printing of colored inks onto light-transmissive media for displaying color images by overhead projection has historically been a problem. When aqueous inks are employed, for example, special coatings must be provided on the light-transmissive medium to absorb the solvent so that images of high quality are formed. See U.S. Pat. Nos. 4,503,111, 4,547,405 and 4,555,437. Even though special coatings are not required on receptor films used for phase change ink jet printing, images produced by prior art color phase change inks printed on light transmissive substrate materials are not generally acceptable for use in an overhead projection system.
The development of phase change inks that are substantially transparent, i.e., inks that transmit substantially all of the light that impinges on them, has improved the quality of images printed on light transmissive substrates. Phase change ink compositions disclosed in U.S. Pat. 4,889,761 are exemplary. Projection of images printed on light transmissive substrates using substantially transparent inks is, however, generally unsatisfactory as a consequence of color ink jet printing techniques.
FIG. 1 illustrates schematically the transmission of light through the central portion of an image printed on a light transmissive substrate. As shown in FIG. 1(a), ink deposited on a light transmissive substrate 14 solidifies as generally hemispherical droplets 12 that refract impinging light beams 10. Refracted light beams 16 are directed away from the collection lens of a projection system (not shown). Light beams 10 impinging on the printed substrate are therefore transmitted through ink droplets 12 in a non-rectilinear path, even if ink droplets 12 are optically transparent. Consequently, the projected image is visible only in contrast, and the colors of the projected image have a dull grayish cast. This problem is exacerbated by subtractive printing techniques wherein multiple layers of droplets are required to produce secondary colors, while primary colors require a single ink droplet.
Another problem that arises in ink jet printing and is evident in projection of phase change ink printed substrates is "banding." As the printer head and substrate move relative to one another and the printer head deposits successive lines of ink, discrepancies arise in the alignment of adjacent printed lines relative to one another. These alignment discrepancies result in the formation of "bands" in the printed pattern at the interfaces of adjacent printed lines. The bands further detract from the appearance and clarity of a projected image.
U.S. Pat. No. 4,889,761 discloses substrates having a light-transmissive phase change ink printed thereon that are processed to improve the quality of images projected by overhead projection techniques. Printed substrates are processed to reorient the surface configuration of solidified phase change ink droplets to provide a printed ink layer having a generally uniform thickness that is capable of transmitting light in a substantially rectilinear path. As shown in FIG. 1(b), light beams 10 impinge on ink layer 20 in a generally rectilinear path, producing collimated transmitted light beams 22 that can be collected by a collection lens of a projection system. Reorientation is achieved by the application of pressure or a combination of heat and pressure to the printed substrate by means of a dual roller assembly Rollers having various constructions are disclosed, including a TEFLON.RTM. coated heated roller and silicone rubber covered pressure roller.
PCT Patent Application No. W0 88/08788 is directed to a method of producing transparencies having curved, light scattering ink droplets printed thereon capable of projecting images. Printed ink droplets are overlaid with a transparent layer having an index of refraction that is substantially the same as the index of refraction of the ink droplets. Preferred coating materials include transparent polyurethane and acrylic. In this manner, the refractive effect of the curvature of the ink deposits is lessened. This publication teaches that the upper surface of the ink covering layer need not be parallel to the substrate surface to achieve this improvement.
European Patent Publication No. 0308117 discloses a transparency having curved, light scattering, colored ink droplets thereon. Exposure of the printed substrate to an elevated temperature of about 70.degree. C.-140.degree. C. achieves spreading and flattening of the ink droplets, but requires a time interval of about 30 seconds to 5 minutes.
Prior art techniques for processing and/or reorienting phase change ink droplets printed on a substrate generally have not provided satisfactory results. Offset problems and problems resulting from the non-uniform distribution of ink droplets persist, especially where multiple ink droplet layers are required. Moreover, most existing processing cycles require unacceptable time periods for completion and thus are not commercially viable alternatives.